Many implements used in material handling and in civil engineering applications have members which are pivotally moved by fluid actuated hydraulic motors or fluid rams. The boom support or swing tower carrying the boom and dipper stick of a backhoe is a typical example. There, a pair of hydraulic motor or fluid rams are used to pivot the swing tower with respect to a fixed support frame or stand. The support frame is usually carried at the rear end of a tractor or similar machine. In such a device the hydraulic motors are ordinarily connected to the swing tower on opposite sides of the vertical pivot axis between the swing tower and the fixed support frame. Thus, when the swing tower is rotated, one of the cylinders initially contracts and the other extends in order to rotate the swing tower. Since the arc through which the swing tower rotates is at least 180 degrees, one of the hydraulic motors applies the primary force to rotate the boom to one side of its midpoint in the arc of rotation while the other hydraulic motor applies the primary force to rotate the boom in the other direction from the midpoint of the arc of rotation.
Simple as the task may be of rotating the swing tower, this problem has confounded engineers and designers of material handling equipment from the very beginning. If the hydraulic motors could be positioned facing each other on either side of the midplane of rotation, each hydraulic motor would apply an equal force across an equal distance to produce an equal moment arm to torque the swing tower about its vertical axis. Because of the spacial limitations imposed upon designers of material handling equipment, both hydraulic motors must be positioned generally parallel to one another. Consequently, in the course of rotating the swing tower from one extreme to the other, each hydraulic motor passes through the plane defined by the vertical axis of rotation of the swing tower and the axis of rotation of that element (i.e., cylinder or piston rod) pivotally connected to the fixed frame supporting the swing tower. Thus, two vertical planes are defined having a common intersection at the pivot axis of the swing tower.
When rotating the swing tower from one extreme to the other, one of the hydraulic motors is driven from a fully contracted position to a fully extended position. The fully extended position occurs when the plane defined by the two pivot axes of the hydraulic motor passes through the vertical axis of rotation of the swing tower. If the swing tower is to continue to rotate, that hydraulic motor must contract in length. When the vertical plane defined by the two pivot axes of the hydraulic motor passes through the vertical axis of rotation of the swing tower, the hydraulic motor is used to pass through its "overcenter position."
Many designers have struggled with this problem. The teachings of J. S. Pilch (U.S. Pat. No. 4,138,928) and E. C. Carlson (U.S. Pat. No. 3,630,120) relate the difficulty in converting rectilinear motion to rotational motion. J. S. Pilch and D. L. Worbach (U.S. Pat. No. 4,085,855) are representative of situations where the same inventor has progressed through a series of patents attempting to reach an optimum solution to this problem. An excellent description of the mechanical aspects of the problem is provided by Arthur G. Short in U.S. Pat. No. 3,872,985.
Thus, there is a long-felt need for a hydraulic circuit which will rotate the swing tower uniformly by the relatively constant application of torque throughout its swing. A simplified, efficient system would be particularly welcomed by the industry.